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Observing Patterns in Inherited Traits
 Knew
that two parents contributed
hereditary material
 Thought

it was blending of fluids
But blending couldn’t explain some traits (a
black horse and white horse should only produce
gray horses)
Austrian Monk, mid-1800s
 Math, physics, botany at University
of Vienna
 Studied Pisum sativum, garden pea
plant

Self-fertilizes—produces both male &
female gametes
 Can cross-fertilize—done by plant
breeders

Observed that white-flowered
parent plants produce white
flowers
 Observed that when bred w/
different-colored plant, different
traits emerged.

 Genes—sequence
of DNA on a chromosome
that gives information on inheritable traits
 Allele—all
forms of the same gene
 Hybrid—offspring
of two “parents” who each
breed true for different forms of a trait
 Homozygous—identical
alleles on homologous
chromosomes
 Heterozygous—non-identical
alleles on
homologous chromosomes
 Dominant
allele—masks effect of recessive
allele when paired


Capital letter for dominant (“A”)
Lower-case letter for recessive (“a”)
 Homozygous
dominant—AA
 Homozygous recessive—aa
 Heterozygous—Aa
 True-breeding parents—P
 First-generation offspring—F1
 Second-generation offispring—F2
 Genotype—particular
alleles in an individual
 Phenotype—Individual’s observable traits
 Sperm
in pollen grains, originate in stamen
 Carpel—female flower part, has eggs,
fertilization, seed development
 Brush pollen onto carpel for artificial
fertilization

Mendel tracked 7 observable traits on pea plants









Seed shape (round or wrinkled)
Seed color (yellow or green)
Pod shape (inflated or wrinkled)
Pod color (green or yellow)
Flower color (purple or white)
Flower position (on stem or at tip)
Stem length (tall or dwarf)
Noted that F1 had all same trait, some of F2 had different trait
Approximately 3:1 ratio of traits

Assume each P was homozygous

AA for one trait, aa for other trait
F1 only show “A” trait, not “a”
trait
 F2 show 1 “a” trait for every 3
“A” traits

 Dominant
(A) genotype suppress recessive (a)
phenotype
 Monohybrid
cross
 Dihybrid
cross
 Heterozygous
homozygotes
phenotype between the two
 Nonidentical
alleles
expressed at same time
 Multiple allele system—3 or
more alleles for a single
gene locus
 Blood types
 Phenotype
results from
interaction among products
of 2 or more gene pairs
 Labrador retriever colors



B (black) & b (brown)
EE or Ee—melanin (pigment)
production. Black or brown
ee—no melanin produced.
Always yellow!
 Responsible
for genetic
variation
 Can result in nonparental
combinations
 The closer the genes on a
chromosome, the more
likely to stay together
 Continuous
variation—range of small
differences in a trait


Due to polygenic inheritance—inheritance of
multiple genes that affect the same trait
Eye color—due to different kinds & amounts of
melanin
 Review
“Summary” & “Genetic Problems” on
pages 182-185….some test questions there